This invention relates to a method for converting oxygenates to olefins. More particularly, this invention relates to controlling the reaction process to maintain a partial pressure-velocity compensation factor of at least 0.1 psiaxe2x88x921hrxe2x88x921 such that a prime olefin selectivity of at least 45 wt. % can be maintained.
Olefins, particularly prime olefins (i.e., ethylene and propylene), have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming desirable feedstocks for making prime olefins. Particularly desirable oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; carbonaceous materials, including coal; recycled plastics; municipal wastes; or any appropriate organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
One way of producing olefins is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using any of a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300xc2x0 C. and 500xc2x0 C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hrxe2x88x921.
It is generally desirable to make prime olefins in a reactor which operates at a high partial pressure of feedstock, since a greater mass of feed stock can be moved through a given reactor size/volume at a given time relative to a reactor operating at a lower partial pressure of feedstock. Alternatively, for a given mass of feedstock to be processed through a reactor, the reactor size at a higher partial pressure of feedstock will be smaller and less expensive relative to a reactor operating at a lower partial pressure of feed stock.
It is also desirable to operate a reactor at a higher weight hourly space velocity (WHSV). Operating at higher WHSVs will enable the reactor volume and catalyst volume to be reduced for a given level of production.
Further, it is generally desirable to operate a reactor using a relatively low proportion of diluent. As the diluent level increases, reactor volume will necessarily increase, without a corresponding increase in feed stock throughput capability. The use of significant quantities of diluent also increases the complexity of the overall process of producing olefins in that the diluent must be separated and recovered, which requires additional facilities in the production process.
Reaching a more desirable level of higher feedstock partial pressure and WHSV, at a relatively low diluent concentration is, therefore, particularly desirable in order to increase the commercial desirability of oxygenates as alternative feedstocks. Unfortunately, in the oxygenate reaction process, increasing the partial pressure of oxygenate to a reactor will oftentimes have deleterious effects on the selectivities of the reaction to desired products, particularly prime olefins, to the point where operation beyond a certain partial pressure is undesirable. Reductions in diluent content of the feedstock may also increase the partial pressure of oxygenate to a reactor, thereby resulting in a decrease of selectivity to prime olefins. Similarly, increasing the WHSV may result in decreased oxygenate conversion. Therefore, operating parameters are needed in order to maintain acceptable levels of prime olefin selectivity in oxygenate conversion processes. Otherwise, the alternative use of oxygenate feedstocks to produce prime olefins will not exceed desirability over conventional petroleum processes.
In order to maintain desirable levels of prime olefin selectivity in catalytically converting oxygenate to olefin product at commercial scale, this invention provides, in one embodiment, a method for making olefin product from an oxygenate-containing feedstock. The method comprises providing a non-zeolite catalyst; and contacting the catalyst in a reactor with an oxygenate-containing feedstock at an oxygenate partial pressure of greater than 20 psia, preferably at least 25 psia, more preferably at least 30 psia.
It is desirable that the catalyst be contacted with the feedstock at a weight hourly space velocity of greater than 2 hrxe2x88x921, preferably in a range of from 5 hrxe2x88x921 to 1000 hrxe2x88x921, more preferably in a range of from 5 hrxe2x88x921 to 500 hrxe2x88x921. It is also desirable that the oxygenate be supplied to the reactor at an oxygenate proportion index of at least 0.5, preferably at least 0.6, more preferably at least 0.7.
The oxygenate-containing feedstock preferably comprises at least one compound selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C4-C20 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof It is particularly desirable that the oxygenate be methanol or dimethyl ether.
The non-zeolite catalyst used in the process preferably comprises a silicoaluminophosphate molecular sieve and a binder. Desirably silicoaluminophosphate molecular sieve is selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, metal containing forms thereof, and mixtures thereof It is particularly desirable that the silicoaluminophosphate molecular sieve be SAPO-34 or SAPO-18, most particularly SAPO-34.
The oxygenate-containing feedstock can be contacted at a wide range of temperatures. Preferably, the oxygenate-containing feedstock is contacted with the silicoaluminophosphate catalyst at 200xc2x0 C. to 700xc2x0 C.
In an alternative embodiment, the average gas superficial velocity is maintained above a minimum level. Desirably, the oxygenate-containing feedstock is contacted with the silicoaluminophosphate catalyst in a reactor at an average gas superficial velocity of greater than 1 meter per second.
In yet another alternative embodiment, this invention provides a method for operating an oxygenate to olefins reaction. The method comprises providing a non-zeolite catalyst; providing an oxygenate-containing feedstock at an oxygen proportion index of at least 0.5; contacting the catalyst and the oxygenate-containing feedstock in a reactor and providing product from the reactor having a prime olefin selectivity of at least 45 wt. %; and maintaining a partial pressure-velocity compensation factor at a level of at least 0.1 psiaxe2x88x921hrxe2x88x921 by controlling weight hourly space velocity and molar flow rate of oxygenate to the reactor.
In a preferred embodiment, the weight hourly space velocity and molar flow rate of oxygenate to the reactor are controlled to maintain a partial pressure-velocity compensation factor of at least 0.15 psiaxe2x88x921hrxe2x88x921, more preferably at least 0.2 psiaxe2x88x921hrxe2x88x921.
In controlling the oxygenate to olefins reaction process it is particularly desirable to operate at a relatively high oxygenate proportion index. Particularly desirable is to operate at an oxygenate proportion index of at least 0.6. Even more desirable is to operate at an oxygenate proportion index of at least 0.7.
The oxygenate to olefins reaction process can be controlled over a wide range of weight hourly space velocities. It is, however, particularly desirable to operate at a weight hourly space velocity of at least 2 hrxe2x88x921. Preferably, the process is operated at a weight hourly space velocity in the range of from 2 hrxe2x88x921 to 1000 hrxe2x88x921, more preferably at a range of from 5 hrxe2x88x921 to 500 hrxe2x88x921.
The invention also includes products made according to the methods provided. The products include the olefin product made directly from the oxygenate reaction process, as well as the derivative products of the prime olefins produced. Polyolefins are particularly desirable products which can be produced from the prime olefins of this invention.
In a particularly desirably embodiment, the catalyst and the oxygenate-containing feedstock are contacted in the reactor and the product from the reactor is provided at a prime olefin selectivity of at least 62 wt. %; and the partial pressure-velocity compensation factor is maintained at a level of at least 0.1 psiaxe2x88x921hrxe2x88x921 by controlling weight hourly space velocity and molar flow rate of oxygenate to the reactor. Preferably, the product from the reactor is provided at a prime olefin selectivity of at least 70 wt. %; and the partial pressure-velocity compensation factor is maintained at a level of at least 0.15 psiaxe2x88x921hrxe2x88x921 by controlling weight hourly space velocity and molar flow rate of oxygenate to the reactor.